Currently, pyroelectric sensors are used in intrusion detection devices to identify intruders. Pyroelectric elements are sensitive to infrared light at wavelengths emitted by the human body, i.e., a wavelength band of about 7 to 25 μm. However, pyroelectric elements are also sensitive to broadband radiation which includes ultraviolet, infrared, and visible light. Much of this radiation is outside the wavelength band emitted by humans. To minimize false alarms, a typical pyroelectric sensing device, used in intrusion detection contains a window (or filter) which filters, i.e., minimizes the transmission of wavelengths, for example, below 5 μm. More specifically, the window is typically formed using a substrate which may be comprised of silicon. Silicon absorbs radiation energy below 1.1 μm and passes radiation energy above 1.1 μm. Filtering of the wavelengths from 1.1 to 5.0 μm is achieved by placing layers of other materials on the silicon substrate. The material in these layers must pass the wavelengths of interest (7.0 to 25.0 μm), while filtering the wavelengths from 1.1 to 5.0 μm. Each material by itself can either absorb or reflect some of the wavelengths not passed.
More specifically, known pyroelectric sensing devices may include a printed circuit board including one or more pyroelectric elements. The pyroelectric elements are electrically connected to a microprocessor. If an electrical signal from the pyroelectric elements satisfies preset conditions, the microprocessor will transmit an alarm signal to an alarm system or monitoring device. The window/filter is formed using a substrate including a plurality of coating layers. The coating layers transmit, reflect, absorb, or cause destructive interference of radiation being focused at the window from a radiation source. A secondary filter may be placed in front of the window such that window is a primary filter working in conjunction with the secondary filter to selectively reflect and pass radiation energy.
Knows pyroelectric sensing devices are inherently susceptible to detecting stimuli not associated with intrusion which results in false alarms and/or false detections. Specifically, pyroelectric sensing devices are susceptible to the radiation energy produced by automobile head lights and other light sources emanating from outside the region being protected, but penetrating into the field-of-view of the pyroelectric device, and ultimately onto the pyroelectric device package. The energy produced by automobile head lamps can be sufficient to cause an alarm in a pyroelectric sensing device. False alarms in intrusion systems are a significant distraction and loss of man hours for the police force, and also can be costly in fines to the owners of the security systems.
Additionally, known intrusion detection systems include passive infrared sensors which detect intruders moving within a field of view by measuring the temperature gradient caused by an intruder. Also, known systems include devices for monitoring a volume of space encompassing a field of view, as disclosed in U.S. Pat. No. 7,145,455, issued to Eskildsen et al. The devices may include a micro electro-mechanical system having mirrors arranged in an array for reflecting IR energy to an IR energy detector which is then converted to an output signal and monitored for determining when an intrusion has occurred. Further, the mirrors are angularly adjusted to detect or cover a desired field of view.
A drawback of these known devices includes the necessity of moving the device or detection system to achieve IR detection in a desired field of view. Further, micro electro-mechanical systems and mirror arrays are expensive to manufacture, as well as, difficult and expensive to maintain and repair.
Additionally, current PIR devices may be built with one or more fixed fields of view designed into the lens array. In the case of multiple fields of view, there is no distinguishing between these different fields of view since each view may cause a non-unique alarm when a sensor detects radiation. Thus, there is no indication of the direction from which the sensed radiation came from or ability to ignore signals from a particular direction.
Additionally, current approaches to solving for false alarms also include augmenting the blocking ability of the pyroelectric detectors window/filter to block unwanted radiation energy. Typically, this includes adding materials, sometimes pigmenting agents (e.g. Zinc Sulfide) to the lens to make the lens more opaque to white light or visible light (energy radiation at wavelengths which the human eye can see) while passing IR (infrared) energy/radiation, or may include addition of a secondary filter. Typically, the amount of a white light absorbing substance added to a passive infrared (PIR) intrusion detector lens to ensure ignoring car headlights is significant, and has an adverse effect on lens transmission in the infrared realm, which may impair the ability of the pyroelectric sensor to detect an intruder. Lens transmission may be reduced by at least 30% in the IR wavelength band between 5 and 25 μm when adequate amounts of pigmentation are added.
Another approach to solving the problem of false alarms is adding a secondary filter to an intrusion detector to ensure that the pyroelectric sensing device ignores car headlights. Secondary filters add significantly to the cost of the intrusion detector and may reduce the IR transmission by approximately 20%. Thus, when intrusion detectors incorporate secondary filters to ensure the pyroelectric sensing device ignores car headlights, the detector may not detect an intruder because the secondary filter reduces the amount of energy that will reach the pyroelectric elements. Further, secondary filters also alter the optical path between each lens element and the pyroelectric elements, which may distort the intended protection.
Additionally, energy between 0.4 and 1.8 μm reaching the pyroelectric detector, for example from an automobile headlamp, is significant and may result in a pyroelectric detector signal sufficient to cause a motion sensor to send an alarm. Specifically, the typical pyroelectric filter does not transmit energy in this wavelength band because the energy is absorbed by silicon and coating layers. However, as the filter absorbs this energy, the energy is converted into heat. This heat is re-radiated at a longer wavelength, passes through the filter and is detected by the pyroelectric element(s). Typical pyroelectric filters used today may contain layers which cause destructive interference in the 1.8 to 5.0 μm wavelength band.
Another drawback to current pyroelectric sensing devices is the susceptibility of the window/filter to absorb energy in close proximity to the sensing elements (ie, the housing and most significantly the optical filter). Although the pyroelectric window/filter blocks energy below 5 μm, a large portion of this blocking comes in the form of energy absorption and a smaller portion from destructive interference and reflection. The absorbed energy is converted into heat, which is re-radiated at wavelengths that pass through the filter to the sensitive pyroelectric elements, thereby generating an electrical response leading to a false alarm from detection of the energy source.
It would therefore be desirable to provide a PIR device and method for intrusion detection which achieves IR detection in a desired field of view without the necessity of moving the device or detection system, or the necessity to reflect or redirect IR radiation to a pyroelectric sensor. It would also desirable to provide a PIR device and method that can prevent unwanted energy from reaching the pyroelectric sensors without producing heat and the undesirable re-radiation of energy. Thus, the desired PIR device would substantially eliminate false alarms/detections without the shortcomings of current devices and methods. It would further be desirable to provide a pyroelectric sensor which prevents visible and near infrared radiation (NIR) energy from reaching the pyroelectric filter. Also, it would be desirable to simplify manufacturing, reduce costs, and improve reliability of current PIR devices.